1. Field of the Invention
This invention relates to a process for controlling the deposition of stickies from paper stock suspensions in papermaking.
2. Discussion of Related Art
Even though paper was invented in the second century, the use of waste material, i.e. the technique of--at least partial--recycling, played a certain part. In today's world, considerable significance is attributed to recycling technology because of the increase in ecological awareness. Accordingly, questions of raw materials supply and the avoidance of waste are becoming increasingly important in the steadily increasing production of paper.
By using secondary fibers through the recycling of waste paper, savings can be now be made in regard to raw materials, waste disposal space and the energy involved in papermaking. However, this technology is attended by specific difficulties.
Thus, in the processing of wastepaper, tacky impurities known as stickies can seriously disrupt the production process and adversely affect the quality of the paper produced. Stickies enter the papermaking process when the wastepaper used contains glued bindings, adhesive tape or specially treated products, such as coated, laminated or coated papers or paperboards. In addition, however, stickies can be formed by the rosin in wood and by its interaction with paper auxiliaries.
If the stickies are present in compact form, they can be chemically removed relatively easily by means of sorting machines. In general, however, the stickies are present not only in compact form, but also in dispersed form in the pulp and are very difficult to remove in this form. Accordingly, the increasing use of wastepaper in papermaking and the reduction in size of the water circuits has increasingly resulted in an increase in the percentage content of stickies in the circuit water.
Stickies cause numerous problems and disruptions not only in the manufacture of paper, but also in the processing of paper. On account of their tackiness, deposits are formed on machine parts, pipe walls, wires, wet felts, dry felts, drying cylinders, smoothing rollers, calender rollers and also in the finished paper, resulting in web breaks in the papermaking machine and in a deterioration in paper quality through holes, specks, marks, etc. (cf. H. L. Baumgarten, Das Papier, 1984, 38, No. 10A, pages V121-V125). According to H. L. Baumgarten, stickies in industrial and institutional publications have for years been regarded as the biggest problem of reusing wastepaper. Even minimal quantities of adhesive can cause breaks in papermaking and printing machines so that stoppages for cleaning are unavoidable. Baumgarten says that "2 g adhesive introduced at an appropriate point of the papermaking machine can result in the rejection of several hundred kg paper" (loc. cit. page V122, right-hand column).
Stickies do not come from a single source. For the most part, they emanate from the rosin in wood, paper-making auxiliaries, binders for the coating of paper and paperboard, paper processing adhesives, printing ink binders and paper processing materials. Of particular importance in the context of the problem addressed by the present invention are the stickies which emanate from the rosin in wood and from the adhesives used in paper processing.
The rosins present in chemical pulp and mechanical pulp contain approximately 1 to 5% by weight so-called harmful resins, depending on the type wood. These resins may be present in colloidally non-bound form or may adhere to the paper fibers. According to J. Weigl et al., the difficulties caused by rosin deposits in the manufacture and processing of paper have steadily increased over recent years for various reasons (cf. J. Weigl et al., Das Papier, 1986, pages V52-V62; more particularly page V53, left-hand column).
The adhesives used in the processing of paper can be divided into three groups, namely: pressure-sensitive adhesives, dispersion adhesives and hotmelt adhesives.
The pressure-sensitive adhesives are permanently tacky products. In their case, adhesion is achieved by application of pressure to the surfaces of the substrates to be bonded. The basic polymers typically used are selected from a number of basic substances in combination with corresponding additives, for example tackifying resins, plasticizers or antioxidants. Typical basic polymers are inter alia natural rubber, butyl rubber, styrene/butadiene copolymers (SBR rubber), acrylonitrile copolymers, polychloroprene, polyisobutylene, polyvinyl ether, acrylates, polyesters, polyurethanes, silicones.
In dispersion adhesives, the polymers used to form the adhesive layer are present as solid particles in an aqueous dispersion medium. In the production process, the basic monomers are first emulsified in an aqueous phase and then polymerized therein--a technique known as emulsion polymerization. The polymer is then present in the form of small particles with different particle sizes which may vary from molecularly disperse to coarsely disperse. In general, agglomeration and, hence, sedimentation of the polymer particles is counteracted by addition of protective colloids or emulsifiers to the system.
So-called hotmelt adhesives or "hotmelts" belong to the group of thermoplastics. Thermoplastics soften on heating and become fluid. On cooling, they solidify again. Examples of polymers used as hotmelt adhesives are polyamides, copolyamides, polyaminoamines, saturated polyesters and ethylene/vinyl acetate copolymers.
Stickies are divided into primary stickies and secondary stickies. Primary stickies are stickies which are not dispersed on account of their high resistance to wet size reduction. Accordingly, they are present in compact form and are easy to remove.
The existence of secondary stickies is attributable to the fact that, in the recovery of wastepaper, the stickies undergo a change in particle size which is brought about by thermal, chemical and mechanical influences. This means that even impurities which are still present in extremely coarse form at the beginning of the recovery process can undergo a more or less considerable reduction in size during the recovery of wastepaper. More particularly, stickies are dispersed by the operations taking place in the hot kneader used for wastepaper recovery. For example, stickies of low melting point are liquefied and then very finely dispersed. Friable or fragile stickies also break up into very small particles. The particle size of the dispersed stickies then extends from coarsely disperse via colloidally disperse to molecularly disperse.
In other words, many stickies are readily dispersible with the result that, after dissolution, they are present in finely divided form and are not picked up in the sorting process. These substances are in danger of forming agglomerates known as secondary stickies in the papermaking machine through thermal, mechanical or chemical influences. It is precisely these secondary stickies which cause problems in the subsequent paper processing cycle. They are transported, for example, with the paper webs, pass through the papermaking machine and thus reach various places where they can cause unwanted deposits, particularly at pressing felts, drying wires, drying cylinders, smoothing rollers. In addition, they do of course also find their way into the finished paper and adversely affect its quality.
Accordingly, it is clear from the foregoing observations that, in principle, any parameters which promote the agglomeration of particles bring with them the danger of formation of secondary stickies. The pH value and the presence of certain papermaking auxiliaries are mentioned as two very important parameters in this regard. More specifically:
Small solid particles in contact with one another or separated from one another by only a very small space attach to one another under the effect of molecular interactions, so-called Van der Waals forces. In general, however, the agglomeration-promoting Van der Waals forces are not developed in alkaline medium, i.e. in the medium typical of wastepaper recovery, because the particles are surrounded by an electrical double layer which is responsible for mutual repulsion of the particles of like charge. By contrast, papermaking machines are normally operated in a neutral or mildly acidic environment so that the repelling negative forces are reduced.
The drainage capacity of paper stock suspensions produced from wastepaper is generally poor. Accordingly, auxiliaries known as drainage or retention agents are frequently used in practice. Retention agents are understood by the expert to be substances which bind fine fibers and fillers to the long paper stock fibers (long fibers). This binding of the short fibers and fillers to the long fibers prevents the fine materials from forming a kind of fleece which complicates drainage of the paper stock suspension. Accordingly, retention agents improve the drainage capacity by binding the fine fibers to the long fibers.
Retention agents can be divided into three groups, namely: inorganic products, such as aluminum sulfate or sodium aluminate; synthetic products, such as polyethylene imines, polyamines or polyacrylamides; and modified natural products, such as cationic starch.
The mode of action of retention agents is based on the attachment of fine fibers and fillers to the paper fibers. An important mechanism in this regard is that polyelectrolytes of sufficient chain length can bridge the distance between two particles and, in this way, cause agglomeration. Thus, J. L. Hemmes et al. report that cationic polyelectrolytes, for example cationic starch, are suitable as trappers for anionic impurities (Wochenblatt fur Papierfabrikation 1993, pages 163-170).
To sum up the situation, it may be said that, so far as experts are generally aware, a neutral or acidic medium on the one hand and the use of cationic auxiliaries on the other hand for drainage and retention displacement represent conditions which promote the agglomeration of particles. So far as the above-discussed problem of stickies is concerned, this means that the expert logically regards these conditions are beneficial to the formation of stickies.
Another key factor in the control of stickies is the temperature. The reason for this is that many adhesives belong to the thermoplastics (hotmelts) of which the tackiness increases with temperature.
In addition, the manifestation of the unwanted properties of stickies for the process of papermaking or processing is dependent on a number of parameters of which the detail is not yet sufficiently known (cf. H. L. Baumgarten, loc. cit., page V122, left-hand column). It is even possible that normally harmless impurities are converted into stickies through the cooperation of mechanical, chemical and thermal influences during the production process (cf. B. Brattka, Wochenblatt fur Papierfabrikation 1990, pages 310 to 313).
Now, there are various known methods which represent attempts to counteract the manifestation of the negative properties of stickies for the process of papermaking. Particular significance is attributed in this regard to the principle of suppressing the deposition of stickies through the use of an auxiliary, so that the disruptions caused by the adhesive properties are reduced to a technically acceptable level. The processes based on this premise are referred to hereinafter as SDC processes ("stickies deposition control").
Thus, U.S. Pat. No. 4,923,566 describes a process in which stickies are controlled by urea.
According to the teaching of U.S. Pat. No. 3,081,219, stickies are controlled in the sulfite pulping of wood by the use of N-vinyl-2-pyrrolidone.
Attempts have also been made to control stickies by the addition of bentonites, diatomaceous earth and the like. This well known principle is based on the idea of introducing fine particles which are capable of binding stickies to their surface (cf. U.S. Pat. No. 3,081,219, column 1, lines 40 to 44). Another premise is based on the addition of sequestering agents, for example polyphosphates (cf. U.S. Pat. No. 3,081,219, column 1, lines 45 to 50). Finally, attempts have also been made to use various dispersants, for example the sodium salts of sulfonated formaldehyde/naphthalene condensates, although this gives rise to disadvantages at neutral pH values and leads to unwanted interactions with cationic auxiliaries (cf. U.S. Pat. No. 3,081,219, column 1, lines 51 to 58).
U.S. Pat. No. 4,744,865 describes an SDC process in which the coagulation of stickies is said to be reduced by methoxyfunctional polymers.
U.S. Pat. No. 4,871,424 relates to an SDC process using hydroxyfunctional polymers. However, only cellulose derivatives, such as hydroxypropyl methyl cellulose, and polyvinyl alcohol obtainable by hydrolysis or partial hydrolysis from polyvinyl acetate are specifically mentioned as polymers.
Finally, G. Galland and F. Julien Saint Amand report that primary acrylate stickies can be removed by flotation in alkaline medium and in the presence of soap (cf. EUR. Comm. Eur. Communities 14011, 1992, pages 235-243). From its very nature, however, this principle cannot assist in solving the problems caused by secondary stickies.